The attenuation control system (i.e. volume control in an audio amplification system) can be a central element of a digital power conversion system.
Most digital controlled power converters are based on a digital modulator that converts a pulse code modulated signals (PCM), received from a source such as a CD-player, to (E.G) pulse width modulated signals (digital PWM modulator). Alternatively the modulator is analog and is preceded by a D/A converter.
The output signal of the digital modulator is fed to a power stage where it is amplified. A typical power converter includes a switching power conversion stage, a filter and a control system.
The digital input of the modulator, i.e. the digital source, can be represented as any given bit frame length, giving a constant dynamic range. An attenuation in the digital domain, as shown in FIG. 1, will compromise the dynamic range of the attenuated digital signal, since the effective bit frame length will be reduced in order to reduce the signal amplitude. Attenuation in the power stage, on the other hand, will not compromise the digital dynamic range.
Noise and distortion contributions in a digital PMA (pulse modulated amplifier) can be caused by inaccuracy of the rising and trailing edges of the pulses in the power stage. An attenuation of the power stage output PWM signal amplitude will retain the dynamic range of the modulated signal, since the noise in the output pulse signals will be equally attenuated.
A general problem in a switching output power stage is the Electro Magnetic Compatibility (EMC), caused by the generally high amplitude of the power stage output PWM signal. In a conventional PMA, the modulation depth is lower at attenuated levels of the digital signal, so that the amplitude of the demodulated signal (i.e. the low pass filtered amplifier output) will be low compared to the amplitude of the power stage output PWM signal. A higher modulation depth combined with lower amplitude of the PWM signal can result in the same modulated signal, but would reduce the EMC problems.
The efficiency of a power stage generally declines when the modulation depth is lowered. This effect is a result of a decreasing ratio between the amplitude of the demodulated, low pass filtered output signal and the amplitude of the power stage output PWM signal. A higher modulation depth combined with lower amplitude of the PWM signal can retain the same modulated signal, but will increase the efficiency and dynamic range.
Therefore, it is desirable to effect attenuation in the power stage, as shown in FIG. 2. A digital amplifier system including attenuation control is described in U.S. Pat. No. 5,898,340. However, this system includes a complex power stage voltage supply, with an output voltage variable in a wide range. A system with a power supply that can change the output voltage continuously within a wide voltage range is very complex and thereby expensive. The mentioned system also includes A/D conversion means in the feedback path from the analog output. This will increase the complexity of the system further.